Electrohydraulic actuator



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Oct. 18, 1966 Filed Sept. 28, 1964 C. W. ASCH E ELECTROHYDRAULIC ACTUATOR 5 Sheets-Sheet 2 II 1111!!!! 1| fit! if l1 IJ Oct. 18, 1966 Filed Spt. 28, 19 64 2 v L 5% E86 ATTORNEY Oct. 18, 1966 c. w. ASCHE 3,279,323

ELEGTROHYDRAULIC ACTUATOR Filed Sept. 28, 1964 5 Sheets-Sheet 5 FIG. 4

INVENTOR. CLARENCE W. ASCHE ATTORNEY United States Patent C 3,279,323 ELECTROHYDRAULIC ACTUATOR Clarence W. Asche, Placentia, Calif., assignor to North American Aviation Inc.

Filed Sept. 28, 1964, Ser. No. 399,454 2 Claims. (Cl. 91-20) The subject invention relates to electrohydraulic servoactuators, and more particularly to multiple control means for improving the operational reliability of electrohydraulic actuators.

In the design and operation of electrically controlled hydraulic actuators, a principal source of unreliability is the electrical signalling network, including the electrohydraulic control valve used to control fluid flow to the hydraulic actuator and the position feedback transducers used for providing closed-loop position control.

Several schemes have been employed in the prior art for improving actuator reliability, all relying upon the use of redundant components. One scheme, as described in U.S. Patent No. 3,070,071, issued December 25, 1962, to A. F. Cooper, discloses an extensible link comprising a plurality of actuators mechanically connected in tandem and commonly responsive to an input signal, whereby the failure of one actuator does not disable the combination, but does reduce both the authority (maximum possible displacement) and the gain (incremental displacement per unit of input signal). Another scheme, as described in U.S. Patent No. 3,095,783, issued July 2, 1963, to C. P. Flindt, employs a plurality of actuators mechanically coupled to an output member, the separate signalling channel for each actuator comprising a position transducer responsive to the common output member for providing a negative feedback position signal to such signalling channel. Where the signalling channel of one of the three actuators malfunctions in such an arrangement, the other two actuators must be able to overpower the first. Hence, the .power rating of each actuator must be 150% of the nominal power rating, and the combined power rating of the three units is therefore 450%. In addition, to the increased power rating of the actuator package, the associated pressurized hydraulic power source (pump, etc.) must be correspondingly increased. In other Words, an arrangement requiring redundant actuators in cooperation with redundant electrical signalling channels for improving control circuit reliability involves a considerable penalty in terms of increased actuator package weight, cost, and space associated with such increased power rating.

The concept of the subject invention provides improved electrical circuit redundancy in an electrohydraulic actuator, while avoiding such weight, cost, space and power penalties.

In preferred embodiment of the invention, there is provided an electrohydraulic servo system having an input control terminal and a common output member, and including at least three independent signalling channels for the transmission of signals from the input control terminal. Each channel comprises an electrohydraulic flow control valve adapted to be connected to a common source of pressurized hydraulic fluid. There is also provided a single hydraulic motor drivingly connected to the output member, and fluid flow summing means responsively coupling the flow-control valves in fluid circuit with the hydraulic motor. There is further included position feedback signalling means responsively coupled to the output member for providing position feedback signals to the signalling channels.

In normal operation of the above described arrangement, each of the control valves provides a controlled fluid flow in response to the difference between the applied input signal and position feedback signal. The combined 3,279,323 Patented Oct. 18, 1966 fluid flow, as provided to the hydraulic motor by the cooperation of the fluid summing means, causes the output member to be actuated in such a sense as to reduce the difference between the input signal and position feedback signal. In the event of failure of one of the control valves or associated signalling channel, the remaining operable signalling channels will continue to provide fluid flow to the fluid flow combining means whereby the actuator is caused to actuate the output member in the proper sense as before.

By means of the above described arrangement, only one hydraulic motor is required, the rating of which need only be of the nominal job rating. Also, no change in the static gain (i.e., of displacement of the output member per unit input signal) results from the failure of a single signalling channel. Further, no change in the hydraulic motor authority, or maximum displacement of the output member, results from such signalling channel failure. Moreover, very little change results in the dynamic response of the servo system, the primary effect being a reduction in the velocity limit due to the limit on the maximum effective fluid flow rate. Further, only one hydraulic motor is required, the rating of which need not be increased above 100%. Accordingly, it is an object of the subject invention to provide an improved electrohydraulic servo system.

It is another object of the subject invention to provide an electrohydraulic actuator having improved reliability.

It is yet another object of the invention to provide redundant control means for an electrohydraulic servo system without increasing either the total power rating or number of hydraulic motors.

It is still another object of the invention to provide an electrohydraulic servo system of improved reliability that is efficient, economical and of limited Weight and volume.

It is a further object of the invention to provide redundant flow control means in combination with flow combining means for achieving increased reliability in the control of a single hydraulic motor.

It is yet a further object of the subject invention to provide redundant means for reliable control of an electrohydraulic servo system without sacrificing the gain and authority thereof.

These and further objects of the invention will become apparent from the following description, taken together with the accompanying drawings in which:

FIG. 1 is a functional block diagram of a system employing the invention.

FIG. 2 is a schematic arrangement of the hydraulic circuit for the device of FIG. 1, illustrating the cooperation between the hydraulic summing means and hydraulic motor for a three way valve arrangement.

FIG. 3 is an isometric drawing, partially torn-away, of a preferred embodiment of the invention.

FIG. 4 is an isometric drawing partially torn-away, of the manifold block of the device of FIG. 3.

In the figures, like reference characters refer to like parts.

Referring now to FIG, 1 there is illustrated a functional block diagram of a servo system employing the concept of the invention. There is provided an electrohydraulic servo system having an electrical input control terminal 10 and a mechanical output member 11. There is further provided three independent signalling channels 12, 13 and 14 for the transmission of signals from the input control terminal 10, each channel comprising an electrohydraulic flow control valve 15 adapted to be :operatively connected to a common source of pressurized hydraulic fluid and responsively coupled in circuit to input terminal 10. There is also provided a hydraulic motor 16 drivingly connected to output member 11. Fluid flow summing means 17 couples hydraulic motor 16 in fluid circuit with valves 15a, 15b and 15c for supplying fluid to hydraulic motor 16. Position piclcolfs 18, 19 and 20, commonly responsive to the motion of output member 11, provide negative feedback position signals to corresponding ones of signalling channels 12, 13 and 14.

Each of signalling channels 12, 13 and 14- is further comprised of signal summing means 21 such as a signal summing amplifier arranged to combine the common input signal (applied to common input terminal and the negative feedback signal from a mutually exclusive one of position pickoffs 18, 19 and 20. Where an A.-C. input signalling ssytem is used and transducers 18, 19 and 20 are A.-C. type transducers, then summing amplifiers 21a, 21b and 210 will further include phase sensitive demodulation means for converting the summed signals to D.-C. signals suitable for use by the electrohydraulic valves 15a, 15b and 150. The construction and arrangement of amplifiers 21a, 21b and 210 are well known to those skilled in the art, and are therefore shown in block or symbolic form only,

In normal operation of the arrangement of FIG. 1, input signals applied to input terminal 10 cause each of flow valves 15a, 15b and 150 to provide a controlled fluid flow of a like sense or direction, in response to input signals applied to input terminal 10. The fluid flow is combined at manifold 17 and fed to hydraulic motor 16 for actuation of output member 11 in a selected sense corresponding to the sense of the controlled fluid flow. Position kickoffs 18, 19 and 20, in response to the motion of output member 11, generate position feedback signals indicative of the position of output member 11 and of a sense convention opposed to that of the input signals applied to terminal 19. The position feedback signals from each pickoif are fed back to the input of an associated one of the summing amplifiers. Each of summing amplifiers 21a, 21b and 210 respond to the algebraic sum of, or amplitude difference between, the two inputs thereto as to produce a fluid flow from the corresponding flow valve 15 to hydraulic motor 16 of such a sense as to cause output number 11 to move in a direction tending to reduce the amplitude difference to zero, as is understood in the art of negative feedback control systems. When the outputs from each of transducers 18, 19 and 20 equals and opposes the applied input from terminal 11, then the resulting flow provided by the associated control valves is zero. The combined flow to motor 16 being stopped, the motor maintains the output member at a position corresponding to the sense and amplitude of the applied signal on terminal 10, the system producing corresponding position changes in output member 11 for subsequent variations in the input signal.

In the event of a malfunction of one channel, such as the failure of a control valve, or feedback transducer or amplifier, the system will still continue to operate as a positional servo. For example, if either or both of amplifier 21a and valve 15a should fail in such a manner as to provide a zero output, the remaining amplifiers 21b and 210 would continue to control the associated valves 15b and 150 for providing a controlled fluid flow to hydraulic motor 16 in accordance with the amplitude difference between the negative feedback position signals and an input signal applied to input terminal 10.

In other words, in the event of a zero signal failure in one of the forward control loops, the system continues to function as a positional servo, without change in either the closed loop gain (output displacement per volt input) or servo authority (maximum achievable displacement). In fact, the system will continue to so operate in the event of zero signal type failures in all but one of the forward loops, so long as such one remaining loop is fully operable.

Another type of failure that may occur is a so-called hard over failure, in which a maximum signal is provided in the forward loop, resulting in a bias flow or maximum flow of a selected sense from, say, valve 15a. Such a failure may occur due to a particular type of failure in valve 15a itself, or to the opening of a feedback resistor in a feedback stage of amplifier 21a in the presence of a small but finite input, or due to the failure of position pickoif 13 in the presence of a finite input signal (other than Zero) on terminal 10. In such event, the resulting motion of output member 11 produced by the bias flow from valve 15:: will produce position error signals at the outputs of corresponding ones of amplifiers 21b and 210 (as well as amplifier 21a). Such position error signals or flow-control signals are fed to valves 15b and which provide controlled fluid flow, which is combined with the bias flow from malfunctioned valve 15a by means of flow summing manifold 17, so as to both compensate for such bias flow and cause motor 16 to drive output member 11 to the proper position. In the steady state, the two fully operational valves 15b and 150 provide a combined compensatory flow which precisely compensates for the sense and magnitude of the bias flow from valve 15a. Associated with such compensatory flow condition, or Zero net flow, is a small displacement error in the position of output member 11, relative to the desired position, which produces a steady state error voltage at the respective outputs of amplifiers 21b and 210 sufficient to provide onehalf the required compensatory flow rate at each of valves 15b and 15c. Such steady-state positional error will normally be negligible for ordinary system gains. Further, where the position servo of FIG. 1 is employed in another (outer) closed loop system, such as an aircraft attitude controller, the effect of such positional error upon the outer loop can be avoided by interpo-sing signal integrating means between such source of the applied input signal (not shown) and input terminal 10, as is well understood in the servo art.

An embodiment of the concept of FIG. 1, employing three-way transfer valves, is shown in FIG. 2.

Referring to FIG. 2 there is shown a schematic arrangement of one embodiment of the invention. There is provided a single hydraulic motor 16 comprising a single cylinder 25 adapted to be connected to a support, and a single movable piston 26 mounted in cylinder 25 and connected to a single mechanical output member 11 extending axially from a first side of piston 26. Such disposition of piston 26 provides first and second chambers 27 and 28 in cylinder 25 on the first and a second side respectively of piston 26, the effective hydraulic area of the first side being preferably one half that of the second side frfzr reasons which will be more fully explained hereina ter.

There is further provided at least three three-way electrohydraulic servo valves 15a, 15b and 15c, each having a pressure input port, controlled flow output port, and a return pressure port. There is further provided an input terminal 10, first, second and third signalling channels 12, 13 and 14, flow-summing manifold 17, and first, second and third position pickolfs 18, 19 and 24), all arranged to functionally cooperate with hydraulic motor 16 in like manner as the like referenced elements of FIG. 1.

Transfer valves 15a, 15b and 150 are similarly constructed and arranged, being three-way elect-rohydraulic servo valves, such as a type commercially available, for example, from Moog Valve Co. of Aurora, New York. Each of valves 15a, 15b and 150 in FIG. 2 has an input pressure port, a controlled flow output port and a return pressure port. The return pressure ports are adapted to be commonly connected to the return pressure side P of a source (not shown) of pressurized hydraulic fluid. Manifold 17 commonly connects the output ports of valves 15a, 15b and 150 to the second chamber 28 of hydraulic motor 16; the second chamber 27 of motor 16 and the input pressure ports of valves 15a, 15b and 150 being adapted to be commonly connected to the high pressure supply side, P of this source (not shown) of a pressurized fluid.

A knowledge of the operation of the three servo valves 15a, 15b and 150 is useful to an understanding of the operating of the arrangement of FIG. 2. The arrangement and operation of valves 15a, 15b and 150 being similar, an explanation of the arrangement and operation of valve 150, shown schematically in FIG. 2, will suffice.

Servo valve 15c is comprised of a hydraulic flow circuit and magnetically actuated means for controlling the hydraulic flow circuit. The fluid control circuit comprises a valve spool 31 slidably mounted in a valve spool cylinder 32 and comprising first and second terminal piston sections 32 and 33 axially spaced from and connected to a center piston section 34 by two intermediate spool sections 35 and 36, whereby the cylinder 31 is divided into two terminal chambers 37 and 38 and two intermediate chambers 39 and 40. The two terminal chambers 37 and 38 and one of the intermediate chambers 39 are commonly connected to the high pressure side P of the source (not shown) of pressurized fluid, the other intermediate chamber 40 being ported to the return pressure side P of the pressurized fluid source. There is also provided two pipettes or restrictive orifices, each in fluid communication with the pressurized fluid of a mutually exclusive one of the terminal chambers 37 and 38 of the valve spool cylinder, and oppositely disposed relative to a magnetically actuated flapper valve 41, in a volume which is connected in fluid circuit with the pressure return line P in a matter not shown. Mechanical feedback is provided between the spool and the flapper valve by means of the interconnection of a fine spring, indicated schematically as element 42.

When the flapper valve 41 is at rest (or unactuated) the valve spool 30 is maintained in the illustrated center position of the valve spool cylinder by feedback element 42, the center cylinder section 34 blocking the annular controlled flow output port. The equal and opposite pressures exerted on the symmetrical hydraulic areas of the piston areas of the spool exert a net force of zero, the

spool being maintained in the center position by the restraining action of the feedback spring 42. In such condition of the valve spool, the neutral leakage or necessary 'minimal leakage flow occurring from the high pressure (P intermediate chamber 39 to the low pressure (P,) intermediate chamber 40 undergoes two pressure drops: one (AP flowing from the high pressure (P intermediate chamber port 39 to the controlled presure (P output port 43, and the other (AP in flowing past the controlled pressure (P output port 43 to the low pressure (P intermediate chamber 40.

AP =P,-P

In the symmetrical condition described, the two pressure drops are equal:

Hence, for the unactuated condition of each of valves 15a, 15b and 150, the common corresponding control pressure P applied to the second chamber 28 of hydraulic motor 16 is one half the line pressure, P Hence, the

6 force F exerted to the right on piston 26 of motor 16 by such pressure (as illustrated in FIG. 2) is equal to the product of the area A of the left side of piston 26 and the pressure applied thereto:

2 This force is just balanced by the force F exerted to the left on piston 26 (as illustrated), which is equal to the product of the system pressure P and the effective area Hence, in the unactuated condition of the valve(s), the forces on piston 26 (of hydraulic motor 16) are balanced, and no motion occurs to output member 11.

Upon the application of an excitation current of a selected sense (from the driving amplifier) to the winding about flapper valve 41, the cooperation of the resulting electromagnetic field with magnetic elements N and S produces a couple or moment which deflects the flapper element towards a selected one of the pipettes, restricting the fluid discharge therefrom. Such restriction in the pipette discharge causes a pressure buildup in the associated terminal chamber of the pressure valve, thereby unbalancing the hydraulic forces on the valve spool 30. Such pressure unbalance or force unbalance displaces the valve spool relative to the annular control flow port, resulting in a change of the control pressure P Such change in the control pressure P is manifested by a fluid flow in a selected direction (i.e., either to or from chamber 28 of motor 16), resulting in motion of member 11 .in a corresponding direction. Such motion continues until the position feedback signal from an associated position transducer is of sufiicient magnitude as to oppose an applied input signal on terminal 10, whereby the actuation current in the flapper valve winding is reduced to substantially zero. In such unexcited state, the flapper valve returns to zero, and the follow-up spring 42 restores the valve spool to the neutral flow position, thereby stopping both the actuation of motor 16 and the motion of output member 11.

In normal operation of the arrangement of FIG. 2,

, all three valves 15a, 15b and would operate similarly and in synchronism. However, should a failure occur to the signalling channel with which one valve is associated, so as to cause zero flow therefrom, the other two valves would continue to function to provide a controlled fluid flow to or from chamber 28 of motor 16, as required, in accordance with the positional difference signal inputs to such valves from the associated ones of driver amplifiers 21a, 21b and 210. If, however, the failure of one signalling channel should result in a bias flow through the associated flow control valve, then the valves of the remaining operative signalling channels would provide a combined compensatory flow to both compensate for such bias flow and actuate motor 16 in the desired direction, in the like manner described in connection with the arrangement of FIG. 1.

For example, should the flapper element 41 for valve 15 become biased to the left, as illustrated in FIG. 2, the throttling of the left pipette would increase the resultant pressure in the left terminal chamber 37 of the valve cylinder, relative to that of the right valve chamber 38, thereby causing valve spool 30 to move to the right. Such motion to the right of valve spool 30 exposes the control valve output port 43 to the pressurized intermediate chamber 39, thereby producing a fluid flow from :port 43 toward manifold 17, which tends to actuate motor piston 26 toward the right. In the absence of a control signal on input terminal 10, the sole input to each of the driving amplifiers associated with the operative valves is the position feedback signal from the corresponding position pickoffs. The response of the operative valves to such input signal is to provide that combined flow necessary to prevent the motion of motor 26 and member 11 (as detected by the pickoffs). Hence, in the steady-state there .is a fluid flow from manifold 17 toward each of the operative valves, the combined flow thereof being equal to the flow from the biased valve. In other words, output member 11 is slightly displaced to the right of the desired or reference position to the extent necessary to .provide a control signal from each of the driving amplifiers 21 of the operative signalling channels, which will produce a compensatory flow of opposite sense and of one half the magnitude of the bias for an associated valve.

If, alternately, the flapper element 41 of the biased valve were biased to the right, the resultant increase of pressure in the right terminal chamber 38 of the valve spool cylinder will urge valve spool 30 to the left (as illustrated in FIG. 2), thereby exposing output port 43 to the pressure-return intermediate chamber 40, resulting in a bias flow from manifold 17 toward control port 43. Such bias flow tends to actuate piston 26 to the left of a reference position, resulting in a positional error signal at the outputs of the driving amplifiers of the operational signalling channels. The response of the operative valves to such error signal is to provide that combined flow necessary to prevent the motion of piston 26. Hence, in the steady-state there is a fluid flow from each of the two operational valves to manifold 17, the combined flow thereto being equal to the bias flow from manifold 17 to the biased valve. In other words, output member 11 is slightly displaced to the left of the desired or reference position to the extent necessary to provide a compensatory flow of opposite sense and one-half the magnitude of the bias, from each of the two operational valves.

A preferred embodiment of a portion of the schematic arrangement of FIG. 2 is shown in detail in FIGS. 3 and 4.

Referring to FIGS. 3 and 4, there is illustrated an isometric view of a preferred embodiment of the actuator shown schematically in FIG. 2, the elements thereof having like reference characters as in FIG. 2 corresponding to like functional elements. A main housing 57, in addition to enclosing the hydraulic motor element 26 and chambers 27 and 28, also houses the three position :pickoff elements 18, 19 and 20 of FIG. 2, one of which is shown partially torn away in FIG. 3. The three translational pickoffs are installed mutually proximate and parallel with the longitudinal axis of the motor (indicated by output element 11) and mechanically coupled to movable element 11 by coupling means 48. For improved resolution, the pickoifs would preferably be an A.-C. inductive type comprising a magnetically-permeable cylindrical slug 54 within a transformer element 55 comprising a primary winding and mutually concentric secondary winding, as described more fully in US. Patent No. 3,136,224 issued June 9, 1964, to A. S. Escobosa. Accordingly, the housing would preferably be constructed of a material combining low magnetic premeability (so as not to interfere with the functioning of the pickoffs) and high strength (so as to be adapted to the high hydraulic pressures usually app-lied to the hydraulic motor). Such a typical material is austenitic stainless steel, Standard Type 303.

Pressure input (P and hydraulic return (P line fittings 60 and 61 are provided near the aft end of the main housing, whereby the device may be operatively connected to a source of pressurized hydraulic fluid. A pressure line manifold 45 commonly connects motor cham- .ber 27 and a manifold block 47 with pressure fitting 60,

while a control manifold line 17 connects motor chamber 28 with manifold block 47. A pressure return line 46 interconnects return pressure fitting 61 and manifold block 47. As shown in FIG. 4, manifold block 47 has a terminal face 50, at least a portion of which sealingly abuts the aft end of the main housing 57 in FIG. 3.

The hydraulic transfer valves 15a, 15b and 15c of FIG. 2 are preferably installed in the manifold block 47 of the device of FIG. 3, which manifold block is shown in greater particularity in FIG. 4. The commercially available configurations of such three-way valves are conventionally cylindrically shaped, with the three hydraulic ports thereof axially spaced along such cylindrical surface. Hence, when the valves are installed in the mating cavities 51a, 51b and 51c provided therefor in manifold block 47, corresponding ports will lie in the same transverse vertical plane taken through block 47 as illustrated in FIG. 4. For example, the pressure return (P ports of the three valves lie in the vertical plane 49 parallel to the terminal face 50 of block 47, and shown in FIG. 4 by partially tearing away block 47. Hence, by milling out a narrow annular groove or cavity 52 in each of valve cavities 51a, 51b and 51c at plane 49, and then ofiset milling'further in such radial directions as to provide an interconnecting cavity between corresponding annular cavities 52, there is provided means allowing fluid communication between corresponding ports of the transfer valves. Then upon drilling through face 50 of block 47, an aperture 46 is provided which may be adapted to sealingly mate with and complete the pressure return line 46, shown in FIG. 3.

Similarly, a control pressure (P manifold 17 and supply pressure (P manifold 45 are provided in block 47 for sealingly mating with and completing the like referenced lines shown in part in FIG. 3.

Electrical connectors (not shown) may be provided for the necessary circuit connections of the valves and position feedback transducers, as is well understood in the art.

Hence, the arrangement of FIGS. 3 and 4 illustrates effective means for implementing the schematic arrangement of FIG. 2.

Although the preferred embodiment of the invention has been illustrated in terms of a hydraulic motor in cooperation with three-way type transfer valves, the concept of the invention is not so limited, and includes the alternative use of four-way type valves, as is understood in the art. Further, although the illustrated embodiment employs a particular type of valve having a valve spool controlled by a single flapper valve, other types or servo valves which include a plurality of flapper elements in cooperation with the valve spool, may be employed without affecting the concept of the invention.

Accordingly, there has been described improved electrohydraulic actuation means having increased reliability; and which is efficient, economical, and of limited weight and volume. Such engineering economy in providing improved reliability, is achieved by the use of flow combining means in combination with a single hydraulic motor.

Although the invention has been illustrated and described in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of this invention being limited only by the terms of the appended claims.

I claim:

1. An electrohydraulic servo system comprising, in combination,

a single hydraulic motor having a mechanical output member;

three position pickoff means in common cooperation with said motor for providing signals indicative of the driven position of said output member relative to said motor;

three summing amplifiers, a first input terminal of each being commonly continuously connected to an input terminal, a second input terminal of each being continuously connected to the output of a mutually ex clusive one of said position pickoif means;

three like proportional type fluid control valves, each continuously responsively connected in electrical circuit to the output of a mutually exclusive one of said amplifiers; and

a manifold block mounting said control valves and commonly connecting corresponding ports of said control valves in fluidcircuit for proportional control of said hydraulic motor in accordance with the combined fluid flow from said valves,

said manifold block having a terminal face which sealingly abuts a main housing of said motor and having cavities for sealingly installing said valves in a mutually-spaced side-by-side and parallel relationship, with corresponding ports of said valves lying in a common transverse plane of said manifold block;

said block further having an annular groove in each cavity at each of said transverse planes, and passages therein radially-extending from said grooves, the passages in a given transverse plane terminating at an intersection thereof as a set of passages for allowing fluid communication between said grooves of said plane;

said block further having a like number of spaced apart apertures located in said terminal face of said block and extending perpendicular to said face, each aperture terminating at an intersection with a mutually exclusive one of said sets of intersecting passages.

2. A single channel electrohydraulic servo comprising a single cylinder adapted to be connected to a support;

a single movable piston mounted in said cylinder, and

connected to a single output member extending axially from a first side of said piston; said piston providing first and second chambers in said cylinder on said first side and a second side respectively of said piston, the effective hydraulic area of said first side being one-half that of said second side;

at least three like three-way electrohydraulic transfer valves, each having an input applied pressure port and a controlled-flow output port; and

a manifold block mounting said valves and commonly connecting respective input and output ports of said valves in fluid circuit with said first and second chambers respectively, said first chamber and said input pressure ports of said valves being adapted to be commonly connected to a source of pressurized fluid,

said manifold block having a terminal face which sealingly abuts a main housing of said motor and having cavities for sealingly installing said valves in a mutually spaced side-by-side and parallel relationship, with corresponding ports of said valves lying in a common transverse plane of said manifold block;

said block further having an annular groove in each cavity at each of said transverse planes, and passages therein radially-extending from said grooves, the passages in a given transverse plane terminating at an intersection thereof as a set of passages for allowing fluid communication between said grooves of said plane;

said block further having a like number of spaced apart apertures located in said terminal face of said block and extending perpendicular to said face, each aperture terminating at an intersection with a mutually exclusive one of said sets of intersecting passages.

References Cited by the Examiner UNITED STATES PATENTS 2,921,562 1/1960 Westbury 91363 X 3,038,449 6/1962 Murphy 91-363 X 3,070,071 12/1962 Cooper 91363 X 3,124,041 3/1964 McMurtry 91-363 X FOREIGN PATENTS 1,191,239 10/1959 France.

MARTIN P. SCHWADRON, Primary Examiner.

SAMUEL LEVINE, Examiner.

P. T. COBRIN, Assistant Examiner. 

1. AN ELECTROHYDRAULIC SERVO SYSTEM COMPRISING, IN COMBINATION, A SINGLE HYDRAULIC MOTOR HAVING A MECHANICAL OUTPUT MEMBER; THREE POSITION PICKOFF MEANS IN COMMON COOPERATION WITH SAID MOTOR FOR PROVIDING SIGNALS INDICATIVE OF THE DRIVEN POSITION OF SAID OUTPUT MEMBER RELATIVE TO SAID MOTOR; THREE SUMMING AMPLIFIERS, A FIRST INPUT TERMINAL OF EACH BEING COMMONLY CONTINOUSLY CONNECTED TO AN INPUT TERMINALS, A SECOND INPUT TERMINAL OF EACH BEING CONTINUOUSLY CONNECTED TO THE OUTPUT OF A MUTUALLY EXCLUSIVE ONE OF SAID POSITION PICKOFF MEANS; THREE LIKE PROPORTIONAL TYPE FLUID CONTROL VALVES, EACH CONTINUOUSLY RESPONSIVELY CONNECTED IN ELECTRICAL CIRCUIT TO THE OUTPUT OF A MUTUALLY EXCLUSIVE ONE OF SAID AMPLIFIERS; AND A MANIFOLD BLOCK MOUNTING SAID CONTROL VALVES AND COMMONLY CONNECTING CORREPONDING PORTS OF SAID CONTROL VALVES IN FLUID CIRCUIT FOR PROPORTIONAL CONTROL OF SAID HYDRAULIC MOTOR IN ACCORDANCE WITH THE COMBINED FLUID FLOW FROM SAID VALVES, SAID MANIFOLD BLOCK HAVING A TERMINAL FACE WHICH SEALINGLY ABUTS A MAIN HOUSING OF SAID MOTOR AND HAVING CAVITIES FOR SEALINGLY INSTALLING SAID VALVES IN A MUTUALLY-SPACED SIDE-BY-SIDE AND PARALLEL RELATIONSHIP, WITH CORRESPONDING PORTS OF SAID VALVES LYING IN A COMMON TRANSVERSE PLANE OF SAID MANIFOLD BLOCK; SAID BLOCK FURTHER HAVING AN ANNULAR GROOVE IN EACH CAVITY AT EACH OF SAID TRANSVERSE PLANES, AND PASSAGES THEREIN RADIALLY-EXTENDING FROM SAID GROOVES, THE PASSAGES IN A GIVEN TRANSVERSE PLANE TERMINATING AT AN INTERSECTION THEREOF AS A SET OF PASSAGES FOR ALLOWING FLUID COMMUNICATION BETWEEN SAID GROOVES OF SAID PLANE; SAID BLOCK FURTHER HAVING A LIKE NUMBER OF SPACED APART APERTURES LOCATED IN SAID TERMINALS FACE OF SAID BLOCK AND EXTENDING PERPENDICULAR TO SAID FACE, EACH APERTURE TERMINATING AT AN INTERSECTION WITH A MUTUALLY EXCLUSIVE ONE OF SAID SETS OF INTERSECTING PASSAGES. 